This application is related to application Ser. No. 622,514 which was filed on June 20, 1984, now U.S. Pat. No. 4,570,710.
In the completion of wells drilled into the earth, a string of casing is normally run into the well and a cement slurry is flowed into the annulus between the casing string and the wall of the well. The cement slurry is allowed to set and form a cement sheath which bonds the string of casing to the wall of the well. Perforations are provided through the casing and cement sheath adjacent the subsurface formation.
Fluids, such as oil or gas, are produced through these perforations into the well. These produced fluids may carry entrained therein fines, particularly when the subsurface formation is an unconsolidated formation. Produced fines are undesirable for many reasons. Fines produced may partially or completely clog the well, substantially inhibiting production, thereby making necessary an expensive workover.
Declines in the productivity of oil and gas wells are frequently caused by the migration of fines toward the wellbore of a subterranean formation. Fines, which normally consist of minutely sized clay and sand particles, can plug and damage a formation and may result in up to a 20-fold, and at times total, reduction in permeability. Conventional sand control techniques such as gravel packing and sand consolidation are sometimes ineffective because fines are much smaller than sand grains and normally cannot be filtered or screened out by gravel beds without a severe reduction in permeability and consolidated sand treatments are restricted to small vertical intervals. In addition, gravel packing and sand consolidation are normally confined to areas surrounding the immediate vicinity of the wellbore. Fines movement, however, can cause damage at points which are deep in the production zone of the formation as well as points which are near the wellbore region.
Normally, these fines, including the clays, are quiescent causing no obstruction to flow to the wellbore by the capillary system of the formation. When the fines are dispersed, they begin to migrate in the production stream and, too frequently, they incur a constriction in the capillary where they bridge off and severely diminish the flow rate.
The agent that disperses the quiescent fines is frequently the introduction of a water foreign to the formation. The foreign water is often fresh or relatively fresh compared to the native formation brine. The change in the water can cause fines to disperse from their repository or come loose from adhesion to capillary walls.
It is well known that the permeability of clay sandstones decreases rapidly and significantly when the salt water present in the sandstone is replaced by fresh water. The sensitivity of sandstone to fresh water is primarily due to migration of clay particles (see "Water Sensitivity of Sandstones," Society of Petroleum Engineers of AIME, by K. C. Khilar et al., (Feb. 1983) pp. 55-64). Based on experimental observations, Khilar et al. proposed a mechanism to describe the dependence of water sensitivity in sandstone on the rate of salinity change.
In most reservoirs, a fracturing treatment employing 40-60 mesh gravel pack sand, as in U.S. Pat. No. 4,378,845, will prevent the migration of formation sands into the wellbore. However, in unconsolidated or loosely consolidated formations, such as a low resistivity oil or gas reservoir, clay particles or fines are also present and are attached to the formation sand grains. These clay particles or fines, sometimes called reservoir sands as distinguished from the larger diameter or coarser formation sands, are generally less than 0.1 millimeter in diameter and can comprise as much as 50% or more of the total reservoir components. Such a significant amount of clay particles or fines, being significantly smaller than the gravel packing sand, can migrate into and plug up the gravel packing sand, thereby inhibiting oil or gas production from the reservoir.
Therefore, what is needed is a method of sand control for use in producing an unconsolidated or loosely consolidated oil or gas reservoir while enhancing the production of hydrocarboneaceous fluids.